The first stage of bacteriophage infection is adsorption to the bacterial host. This critical step, which determines the host spectrum of the phage, is performed by specialized proteins attached to the phage tail in the form of fibers or more globular spikes. The putative receptor-binding part of long tail fibers of most T-even bacteriophages (e.g. T2, T6, RB32) consists of two proteins – a long rod-like gp37 and a small gp38, which is thought to cap the tip of gp37 and play a critical role in host selectivity. Notably, in the best-studied phage of this group –T4 – gp38 is not a host spectrum determinant, but a chaperone for correct fiber folding.

To unravel how the receptor-binding complex of T-even phages is organized and functions, we analyzed structural and functional aspects of interaction of long tail fibers of T2-like phage S16 with Salmonella. S16 is unique in its ability to infect a wide range of Salmonella serovars making its receptor-binding fibers particularly interesting. The receptor-binding tip of S16 long tail fiber, which consists of a C-terminal fragment of gp37 and complete gp38, can be used for immobilization, enrichment, and detection applications of clinically relevant Salmonella serovars.

We report here the crystal structure of the putative receptor-binding tip of the long tail fiber of bacteriophage S16. Gp37 proteolytically auto-cleaves, forms a trimer, and presents a flat triangular platform for binding of a single copy of gp38. The latter consists of three domains with the least conserved one forming the distal tip of the fiber. This domain represents a feat of protein folding and contains never-seen-before stretches of fully ordered glycines (e.g.151-GGGGGGGG-158) in the extended conformation that form a beta-sheet sandwiched between two other beta-sheets. Our results explain structural organization of T-even long tail fibers and create a framework for fiber engineering and applications.